1. Field of the Invention
The invention relates generally to the field of determining orientation and electrical resistivity of subsurface rock formations using measurements made from within a wellbore. More particularly, the invention relates to methods for determining such attitude and resistivity using combinations of multiaxial induction measurements and wellbore image measurements.
2. Background Art
Important parameters for evaluating the structure and the circumstances of geologic creation of subsurface rock formations include the geodetic attitude or “dip” of the rock formations. Dip is usually defined as the geodetic orientation of a rock formation along the direction corresponding to layers of the rock formation. Other important parameters include electrical resistivity of the rock formations, which is related to fractional volume of pore space in the rock formations and the fluid content in the pore spaces of such rock formations. In some cases, rock formations consist of relatively thin (with respect to the axial resolution of certain measuring instruments) layers of rock having alternating high and low electrical resistivity. Determining electrical resistivity of the more resistive layers in such formations has proven useful in their identification and their characterization as containing useful materials such as oil and natural gas.
Methods are known in the art for evaluating dip of rock formations using measurements made from within a wellbore drilled through the rock formations. One such method includes making electromagnetic induction measurements of the rock formations along mutually orthogonal electromagnetic field directions from within a wellbore. An instrument for performing such measurements and for evaluating dip and electrical resistivity of the formations from such measurements is used to provide services under the service mark RT SCANNER, which is a service mark of an affiliate of the assignee of the present invention. Dip and resistivity evaluation using the RT SCANNER instrument provides information on a geologic structural scale, that is, on the order of entire layers of rock formation usually 1 meter or more in thickness.
Another technique for evaluating rock formation dip known in the art includes evaluation of resistivity measurements made along the wall of the wellbore using a device having a plurality of galvanic measuring electrodes spaced on “pads” configured to contact the wall of the wellbore. Current flow and/or voltage drop with reference to selected ones of the electrodes is related to electrical resistivity of the rock formations in contact with the pads. Resistivity as determined from current flow and/or voltage drop may be used to calculate a color or gray scale image density value corresponding to each of the electrodes, and such color or gray scale image density values may be presented in a display that corresponds to circumferential position of each of the electrodes on the wellbore wall and the depth of each of the electrodes along the wellbore wall. Such presentation corresponds to a visual “image” of the wellbore wall. One instrument that is used to make such measurements provides services sold under the service mark FMI, which is a service mark of an affiliate of the assignee of the present invention. Dip may be estimated using such images by estimating axial displacement of rock formation layer boundaries from one side of the wellbore to the other, and converting such axial displacement into angular displacement of the rock layers from geodetically horizontal and angular orientation with respect to a geodetic or other reference. Formation layer boundaries may be inferred from the image, either automatically by setting image density thresholds, or by visual inspection of the display. Dip estimation using the FMI service can have resolution to a scale of several millimeters. Limitations to the accuracy of such dip estimation using the FMI service result from the fact that converting the layer position displacement to geodetic angular displacement assumes that the wellbore is round and that the image values correspond to resistivity changes at the wellbore wall surface. As will be appreciated by those skilled in the art, the actual resistivity response of a galvanic electrode device may be laterally displaced from the wellbore wall by an amount related to the electrical resistivity itself, and in many cases, the wellbore is not perfectly round, but may be oval shaped, or rugose, depending on the particular rock formations that were penetrated and fluid used to drill the wellbore, among other factors.
Measurements made by instruments such as the RT SCANNER instrument are typically processed by an inversion technique to provide “vertical” resistivity, “horizontal” resistivity and apparent formation dip. Vertical resistivity is generally defined as electrical resistivity measured in response to electric current flowing in a direction normal to the formation layer boundaries, while horizontal resistivity is generally defined as electrical resistivity measured in response to electric current flowing generally parallel to the layer boundaries. The foregoing multiaxial induction inversion technique assumes a relatively simple geometric model for the rock formations, namely that they are disposed in substantially infinitely extending, parallel layers (the so called “layer cake” model). Such model generally does not take into account effects of lateral variations in the formation resistivity caused by, e.g., wellbore fluid infiltration into porous rock formations (“invasion”), fractures, cross-bedding, lateral variation of rock formation composition and/or grain size distribution (“facies”), formation layer termination by reduction of thickness (“pinch out”) laterally away from the wellbore, and rock formation heterogeneities such as inclusions of pebble size grains, precipitated quartz (“chert”) and/or other minerals.
Therefore, differences exist between the dip determined using image instrument measurements and the measurements from the RT SCANNER instrument. Such differences can be explained based on the principles of measurement of each instrument, but the explanation will vary depending on the particular rock formation and the wellbore shape. Understanding such differences can results in a more accurate dip determination. Still further, accurate evaluation of formation dip may be used to improve the results of the multiaxial induction inversion procedure by removing dip as an output parameter of the inversion procedure. There continues to be a need for improved rock formation dip determination.